1. Field of the Invention
The present invention relates to a color image forming apparatus and an image deviation correcting method for correcting a deviation of an image writing position and a deviation of an image magnification.
2. Description of the Related Art
Color image forming apparatuses form a color image by superimposing respective colors, unlike the case of forming a monochrome image. When positions of the colors are deviated, colors of line images and characters are changed, and irregular colors are generated in the image, thereby deteriorating the image quality. Therefore, image positions of the colors need to be matched.
Image forming apparatuses that use a light beam scanning device modulate a light beam with image data, and rotate a polarizer (hereinafter, polygon mirror), thereby polarizing the light beam at an equal angular speed in a main scan direction. The image forming apparatus corrects the equal angular speed polarization to an equal speed polarization with an fθ lens, and scans the image on an image carrier (hereinafter, photoconductor).
However, in the conventional technology, image magnification differs for each machine due to variations in characteristics of the light beam scanning device (lens). Particularly, when a plastic lens is used, the shape and the refraction index of the plastic lens change due to a change in the ambient temperature, a change in the machine temperature, and the like. Consequently, a scan position on the image surface of the photoconductor changes, and a magnification error occurs in the main scan direction. As a result, a high-definition image cannot be obtained. In apparatuses that form a color image using plural laser beams and lenses, since color registration deviations occur due to magnification errors, a high-definition image cannot be obtained. Consequently, image magnifications of the colors need to be matched as much as possible.
Techniques of correcting position deviations in the main scan direction due to various factors such as a change in the ambient temperature and a change in the temperature within the image forming apparatus that forms color images using plural photoconductors, are disclosed in Japanese Patent Application Laid-Open Nos. H8-146827, H11-198435, and 2000-355122.
Japanese Patent Application Laid-Open No. H8-146827 discloses a relatively easy and highly precise technology for correcting the magnification of an image in the main scan direction and correcting distortions of other images relevant to the correction of the magnification. Specifically, frequencies of video clocks of colors are changed to obtain an equal interval between two points for each color, and an image writing start position and an image writing end position are corrected, thereby decreasing color registration deviations.
Japanese Patent Application Laid-Open No. H11-198435 discloses a technology for correcting a position deviation of an image formation position without complicating the configuration of the apparatus. Specifically, each time an image is scanned once with a light beam, a frequency controller changes the frequency of a clock signal by a width corresponding to a recording magnification of a part of an assigned image based on a frequency corresponding to a recording magnification of the whole assigned image, thereby correcting a deviation in the recording magnification of the total image and a partial variation in the recording magnification of the image.
Japanese Patent Application Laid-Open No. 2000-355122 discloses a technology for correcting a main scan magnification in high precision at low cost without increasing a frequency jitter of an image clock and substantially decreasing an image deviation at a part where a sub-pixel is added. Specifically, an image clock width corresponding to several pixels is increased or decreased in an image area, thereby matching the total image width in the main scan direction.
Regarding the color registration deviation in the main scan direction, it is necessary to match the writing start position, the writing end position, the total magnification, and the partial magnification for each color. Even one deviation (error) will lead to a color registration deviation. Therefore, the position and the magnification for each color need to be adjusted with high precision. For an apparatus that forms an image position deviation detecting pattern and detects and corrects a deviation among respective colors, it is preferable to carry out all corrections by a single operation, from the viewpoint of toner consumption and a printing speed. However, according to Japanese Patent Application Laid-Open Nos. H8-146827, H11-198435, and 2000-355122, the corrections cannot be carried out by a single operation. Furthermore, according to the method of variably controlling the frequency of the pixel clock and correcting the total magnification and the partial magnification while the light beam is scanning in the main scan direction, it is very difficult to obtain, especially at low cost, a stable frequency when a phase-locked loop (PLL) circuit is used.